As line widths and features within the semiconductor industry continue to decrease in size there is an ever-increasing need to discover new ways and tools to accurately define the size and shape of the features in a microcircuit. Critical dimensions and accurate formation of various devices within an integrated circuit are paramount in producing high quality semiconductor devices, and the scanning electron microscope (SEM) has long been an industry standard for examining such features. The SEM uses a very fine probing beam of electrons that sweeps over the surface of the specimen causing the surface to emit a variety of radiations. Measuring the radiation creates a signal that is proportional to the amount of radiation leaving an individual point of the specimen at any instant. This signal can be used to modulate the brightness of a display cathode-ray tube (CRT) as an illumination beam rests on a corresponding pixel of the CRT image. In practice, the pixels follow one another with great rapidity so that the image of each pixel becomes an image of a line, and the line, in turn, becomes a series of lines that move down the screen so rapidly that the human eye sees a complete image. The CRT image can also be recorded in its entirety by allowing the pixel-by-pixel information to build up in sequence on a photographic film.
As semiconductor features continue to decrease in size, now reaching less than 200 nm and projected to reach to about 120 nm, it is becoming increasingly important to have the ability to measure the actual features formed on a semiconductor wafer. The SEM has historically been an excellent analytical tool for determining the nature, width, and length of features on the upper surface of a semiconductor die. In the early 1990's the SEM was adequate for detailed feature analysis because feature size was on the order of 500 nm and larger. As feature sizes continue to decrease, the exact nature of the sidewall becomes increasingly important. However, a SEM beam that is vertical, i.e., with respect to the die surface, has significant difficulty in determining the depth of some features in today's sub-250 nm feature sizes.
To illustrate the problem of a vertical SEM on a very small surface, refer initially to FIG. 1. Illustrated is a sectional view of one embodiment of a simple semiconductor wafer feature 100 being subjected to a vertical electron beam, collectively 105, of a SEM (not shown). The illustrated semiconductor wafer feature 100 comprises, a first upper surface 111, a lower surface 120, first and second sides 131, 132, and a second upper surface 112. The first and second sides 131, 132 are shown as they are typically found. That is, the sides 131, 132 are not exactly vertical, but rather slightly outward sloping (note angles 131a and 132a), with respect to the lower surface 120. In prior years, the wall slope, i.e., typically angles 131a, 132a of perhaps 0.5 to 3 degrees off the vertical, of channel features was known, but was not significant when considered against a total width 101 and depth 102 of the feature 100.
While the planar location (x and y coordinates) of any point on a surface of a feature can readily be ascertained from the stepper mechanism that operates the electron beam 105, the vertical location (z coordinate) of the point may be problematic. As the vertical electron beam 105 of a SEM passes from left to right, i.e. passes through positions 105a through 105m sequentially, the first upper surface 111 is readily defined by the beam 105 at positions 105a through 105c. However when the electron beam 105 passes over the first side 131, that is, from 105d through 105f, there can be an uncertainty as to the depth of the surface 131 being impacted by the electron beam 105. An edge effect causes secondary electrons 106 to be generated when the electron beam 105d, 105e strikes a corner 133 of sloping first side 131 and causes what is called a "blooming effect" in the image. As with the first upper surface 111, the lower surface 120 is readily discerned by the electron beam 105g-105i, but the blooming effect re-occurs on the second side 132 at positions 105j through 105l. This disrupts how finely the sidewall 131, 132 depth can be determined. With wall slopes as mentioned, the morphology of the wall where the electron beam 105 is striking becomes clouded as the secondary emission 106 of electrons from the target blooms. It therefore becomes uncertain as to the exact shape and dimensions of the side walls 131, 132.
Thus, a vertical SEM is limited in usefulness for analyzing an existing feature. To effectively use the SEM for feature depth measurements, the semiconductor die must be sectioned, allowing SEM to be performed on the section, rather than vertically from the upper surface. This allows what could be termed a horizontal SEM, i.e., a SEM oriented into the plane of FIG. 1. However, sectioning results in destruction of the wafer, and is therefore undesirable.
Another negative factor with SEM examination is that it charges the surface being examined, that is, electrons are bombarded onto the surface of the sample, and secondary emissions from the target are then measured. Thus, the scanning electron microscope has about reached its limit in its ability to provide information on the semiconductor features being formed today. Therefore, one might reasonably prefer to have a non-intrusive examination method that does not interact with the sample or its surface.
In light of the aforementioned problems, one approach to a solution might be to use a physical measurement system, bypassing the intrusive nature of the SEM, as well as eliminating a need for sectioning the semiconductor die. One such existing tool is a stylus nanoprofilometer (SNP), also know as a critical dimension atomic force microscope (CDAFM). Referring now to FIG. 2, illustrated is a schematic representation of a conventional, single-direction balance beam force sensor 200. The SNP (not shown) uses the balance beam force sensor 200 to monitor a force 215 between a probe tip or stylus 210 and a sample surface 220. Additional information on balance beam force sensors may be obtained in "Dimensional Metrology with Scanning Probe Microscopes", Journal of Vacuum Science and Technology Bulletin 13, pg 1100, pub. 1995, incorporated herein by reference. By monitoring a change in capacitance at locations 251, 252 between a scan actuator 230 and a balance beam 240, contact with the surface 220 can be detected. Referring now to the enlarged view, by moving the probe tip 210 from point to point on the sample surface 220, one who is skilled in the art will readily understand that the topography of the surface 220 can be mapped.
Of course, different problems present themselves when using physical means, rather than a SEM, for device measurements. For critical dimension measurements, the shape of a mechanical probe tip, which is of course a finite size, must be extracted from the obtained data. Therefore, mechanical probe tips must be: (a) made so that they are easily characterized, and (b) have only one proximal point, that is, one point of interaction between the sample and the probe tip.
Referring now to FIGS. 3A and 3B, illustrated are elevational views of probative portions 311, 321 of conventional cylindrical and conical probes 310, 320, respectively, for a stylus nanoprofilometer. In the early 1990's, cylindrical and conical probe tips could be made by chemically etching a single strand of optic fiber to form a terminus width 312 of about 500 nm with a length 313 of about 1000 nm. At the time, these probe tip dimensions and the cylindrical or elongated conical shapes 311, 321 were adequate for the topologies of semiconductor features then being formed.
In everyday life physical measurement with tools, e.g., dial indicators, thickness calipers, etc., employing a tapered probe tip is very common. Accordingly, one might believe that a conical probe tip only interacts with an intended surface 330 at the very extreme end or terminus 314, 324 of the tip 310, 320, respectively. Of course, when working in sub-200 nm dimensions, one who is skilled in the art will readily understand that it may be, for all practical purposes, impossible to shape a terminus 314, 324 on the order of 1 nm in width. Therefore, the terminus 314, 324 must realistically have some finite thickness.
For critical dimension measurements, the shape of the probe tip 311, 321 must be extracted from the obtained scan data. With a conventional probe tip 311, 321, surface features 340 that are small in relation to the probe tip 311, 321 "reflect" the probe tip 311, 321. That is, as the probe 310, 320 moves along from left to right, a relatively large probe tip 311, 321 contacts the feature 340 at multiple times, e.g., contact points 371, 372, 373, and 381, 382, 383, respectively, thereby reflecting the shape of the tip 310, 320. Therefore, probe tips 311, 321 must be made so that they are easily characterized and have only one proximal point, i.e., one point of interaction between the sample and the tip. Of course, as FIGS. 3A and 3B show, the tips 311, 321 actually have several proximal points 371, 372, 373 and 381, 382, 383, respectively.
Referring now to FIG. 4, illustrated is a conventional conical probe tip 410 in relation to a sectional view of a semiconductor feature 400 of high aspect ratio. Although the reflection effect of the conical tip is significantly less than the cylindrical tip, it may not always be possible to ascertain the presence or absence of material near the sidewall of a feature with a conventional conical probe. For example, it is extremely important to know if all of a photoresist layer, e.g., 421, 422, has been removed during a stripping process. When the conical probe tip 410 with a positive sidewall angle 415, i.e., a tip with a conventional conical point, encounters small features of high aspect ratio, the sidewall angle 415 of the probe tip 410 limits the ability to obtain data from areas 431, 432. This occurs because a tip surface 412 encounters a corner 401 of the tip 410 first. Because of the high aspect ratio of the feature 400, photoresist material may still be present in the areas 421, 422 deep in the feature 400.
Accordingly, what is needed in the art is a non-destructive measurement system for semiconductor features that avoids the limitations of the aforementioned measurement systems.